1. Field of the Invention
The present invention generally relates to the field of the recovery of valuable material from industrial waste. More particularly, the present invention relates to the field of the recovery of neodymium (Nd) from industrial by-products of rare earth magnet manufacture.
2. Description of the Prior Art
There are two waste materials which are by-products of the rare earth magnet manufacture. Both waste materials contain iron (Fe) and rare earth metal neodymium (Nd), in the form of Nd.sub.2 Fe.sub.14 B, where B is boron. The first is a powdery grinding swarf (hereafter "NdFeB swarf"). The second is a nodular metallic slag (hereafter "NdFeB slag"). The two waste materials require two considerably different approaches to chemical schemes for recovering the neodymium (Nd) values contained therein. However, the objective behind both approaches are the same: to obtain maximum product purity, while totally avoiding the generation of problematic new waste materials, and maintaining work place and environmental safety as well as cost effectiveness.
In the prior art, the process of recovery of neodymium (Nd) from the by-products of rare earth magnet production are not satisfactorily developed and implemented on an industrial scale. There are two major problems in the development of a feasible process for the recovery of neodymium (Nd) from either the NdFeB swarf or the NdFeB slag. The first problem is the high cost of chemical material incurred which the processing of the constituent iron demands. The second problem is the massive disposal of the considerable mass of the iron by-product in some chemical form or other.
The difficulty which persists in the prior art for neodymium (Nd) recovery is both theoretical and practical. As a practical difficulty, the iron is intimately dispersed within the magnet alloy and the entire waste mass must be solubilized to separate neodymium (Nd), and there is just no simple way to do that in the prior art. As a theoretical difficulty, it has been conventionally believed in prior art rare earth chemistry that the lanthanide metals would liberate hydrogen from water and be attacked by acids, but not by alkalis. This belief has effectively foreclosed any attempt in utilizing alkalization in a neodymium (Nd) recovery process. As a result, prior art neodymium recovery processes are almost exclusively based on acid dissolution at the initial stage of the recovery process.
J. W. Morrison and G. R. Palmer, chemical engineers in the Salt Lake City Research Center of the Bureau of Mines, United States Department of the Interior, wrote an article entitled "Recovery of Metal Values From NdFeB Magnet Scrap" (hereafter "the Morrison article").
The Morrison article mentioned several prior art methods of neodymium recovery, such as magnetic and leaching procedures, but discarded them because "the extremely fine grain size of the oxidized scrap prevented recovery by either technique" (page 1, Abstract). It disclosed a recovery process of using sulfuric acid (H.sub.2 SO.sub.4) dissolution followed by precipitation of neodymium-sodium-sulfate double salts having formulas such as Nd.sub.2 (SO.sub.4).sub.3 .multidot.Na.sub.2 SO.sub.4 .multidot.6H.sub.2 O, Nd.sub.2 (SO.sub.4).sub.3 .multidot.3Na.sub.2 SO.sub.4 .multidot.126H.sub.2 O, or NaNd(SO.sub.4).sub.2 .multidot.nH.sub.2 O, where n is 0 or 1 (page 14, lines 11 through 13). The neodymium-sodium-sulfate double salts were then treated with hydrofluoric acid (HF) to produce neodymium trifluoride (NdF.sub.3). The major problem with the prior art sulfuric acid based processes is that the neodymium-sulfate salts do not crystallize properly from aqueous solution, and cannot yield a pure product.
The final treatment of neodymium trifluoride (NdF.sub.3) has also presented some problems in the prior art. The converted neodymium trifluoride (NdF.sub.3) contains a significant amount of moisture and must be dried. In the prior art the drying step is normally carried out by heating the neodymium trifluoride (NdF.sub.3) in the atmosphere of hydrogen fluoride (HF) gas. This presents considerable work place hazard, and an air-pollution control problem as well. Another prior art drying method is thermal-drying. However, the thermal-drying method tends to eliminate the hydrogen fluoride (HF) gas with the formation of neodymium oxyfluoride (NdFO), which is unacceptable as a feed material to the calciothermic metal winning method. An alternative prior art drying method is air-drying. Unfortunately, air-dried neodymium trifluoride (NdF.sub.3) contains about at least 3% moisture, which is not quite acceptable since the calciothermic process in which it is used to make industrial neodymium metal is highly sensitive to moisture.
It is highly desirable to provide a simple and advanced recovery process for industrial scale operation, which process can produce an optimum neodymium (Nd) recovery rate, while conforming with very high environmental and work place safety standards, and maintaining very low production costs.